(i) Field of the Invention
The present invention relates to a method for producing a gallium nitride-based compound semiconductor on a substrate.
(ii) Description of the Related Art
Gallium nitride-based semiconductors are widely used in luminous devices such as LEDs, and in other applications. In production of GaN semiconductors, a blue laser which is operable continuously over 10,000 hours at room temperature has been reported (S. Nakamura et al, Apply.Phys.Lett.72, 211(1998)) for growing GaN on a sapphire substrate by using an ELO (Epitaxially Laterally Overgrown) method. In the ELO method, a GaN layer of several microns is formed on the sapphire substrate and an SiO2 mask is formed in the form of stripes in the  less than 1100 greater than  direction of GaN. SiO2 formed in the form of stripes has an opening ratio of about 2:1. GaN is caused to grow again in a perpendicular direction from the openings of the SiO2 mask, after which GaN is caused to grow in a horizontal direction so as to cover the SiO2, thereby forming a continuous GaN layer. Thus, the dislocation density of the GaN layer covering the SiO2 mask is reduced and a luminous device having the above properties is obtained.
However, in this ELO method, a reduction in dislocation density occurs only at the portions of the GaN layer where the SiO2 mask exists, and only these portions exhibit good properties.
Meanwhile, in view of lattice mismatching between sapphire and GaN, it has also been proposed that a GaN or AlN buffer layer be grown on a sapphire substrate at low temperatures and then a GaN or GaAlN layer or the like be grown on that buffer layer. For example, Japanese Patent Application Laid-Open No. Hei 4-297023 discloses that a GaAlN buffer layer is caused to grow on a sapphire substrate at low temperatures and then a semiconductor layer such as GaN is further formed.
However, even in this method, high dislocation density occurs in the low-temperature buffer layer, so that high dislocation density also occurs in the GaN or GaAlN layer formed thereon. This is not satisfactory in obtaining a luminous device that is operable continuously over a long period of time.
Among the objects of the present invention are to provide a method which enables relatively simple reduction of dislocation density of a GaN-based compound semiconductor to a level at least that resulting from the ELO method, and to provide a GaN-based compound semiconductor device having low dislocation density.
To achieve the above objects, the present invention relates to a method for producing a GaN-based compound semiconductor, which comprises the steps of (a) forming a buffer body discretely on a substrate, (b) forming buffer layer on the buffer body, and (c) forming a GaN-based compound semiconductor layer on the buffer layer.
One aspect of the present invention further includes a step of forming a crystal nucleus generation-inhibiting layer discretely on the substrate prior to the step (a).
Another aspect of the present invention further includes a step (d) of forming an InGaN layer on the GaN-based compound semiconductor layer.
Still another aspect of the present invention further includes a step (d) of forming a superlattice layer having a quantum well structure on the GaN-based compound semiconductor layer.
Further, in the present invention, the step (a) can also be expressed as a step of forming a buffer body having multiple pores on a substrate.
Still further, in the present invention, the step (a) may also be a step of feeding SiH4 and NH3 to a substrate. In one aspect, the amounts of SiH4 and NH3 to be fed are such that an Si compound is formed in the form of islands on the substrate.
In the present invention, when a buffer layer is formed on a substrate (at relatively low temperatures) and a GaN-based compound semiconductor layer is further formed thereon, it is possible to reduce the dislocation density of the GaN-based compound semiconductor layer formed on the buffer layer if the dislocation density of the buffer layer can be reduced. The present inventor has found that by forming a buffer body discretely on a substrate, by forming a buffer body having multiple pores on a substrate, or by forming a buffer body in the form of islands on a substrate, buffer layer crystal growth, which is dependent on the substrate, is inhibited over the buffer body to suppress the occurrence of dislocations, and monocrystallization is promoted to generate seed crystals of a GaN-based compound semiconductor. More specifically, the buffer layer grows in a perpendicular direction from the pores of the buffer body formed discretely and, in time, the buffer layer grows in a horizontal direction so as to cover the buffer body. Growth of the buffer layer may be interrupted in the perpendicular direction from the pores, but not in the horizontal direction because the buffer layer is free from substrate influence. By controlling the degree of discretion of the buffer body or the number of its pores, the degree of crystal growth in the horizontal direction can also be controlled.
In one embodiment of the present invention, a buffer body is formed when the temperature of a substrate is 900xc2x0 C. or lower. When a buffer body composed of a material such as SiN is formed at a substrate temperature of higher than 900xc2x0 C., the surface of the substrate is automatically nitrided, and the buffer body is inevitably formed on the surface-nitrided substrate. It is well known that when GaN is formed on the surface-nitrided substrate, the quality of the GaN layer formed is adversely affected. Therefore, it is desirable that formation of the buffer body be carried out at temperatures at which the surface of the substrate is not yet fully nitrided, which are specifically 900xc2x0 C. or lower, preferably 700xc2x0 C. or lower, and more preferably 450xc2x0 C. to 600xc2x0 C.
In the present invention, a reduction in the dislocation density of the GaN-based compound semiconductor may cause distortion in the grown GaN-based compound semiconductor. When the GaN-based compound semiconductor is caused to grow directly on the substrate, dislocation occurs. This dislocation alleviates the distortion of the GaN-based compound semiconductor. Such distortion does not become noticeable when the thickness of the GaN-based compound semiconductor layer is very small, but when the distortion increases in proportion to an increase in the layer thickness, problems such as layer cracking may occur.
Thus, in the present invention, a crystal nucleus generation-inhibiting layer is formed discretely prior to the formation of the buffer layer, not only to reduce the dislocation density of the GaN-based compound semiconductor, but also to ensure the alleviation of its distortion. The GaN-based compound semiconductor grows from the portions of the substrate where the crystal nucleus generation-inhibiting layer is not formed and, in time, the growing portions of the GaN-based compound semiconductor proceed in a horizontal direction and meet one another so as to cover the crystal nucleus generation-inhibiting layer. At these meeting portions, the distortion of the GaN-based compound semiconductor layer is alleviated.
Further, in the present invention, an InGaN layer or a superlattice layer having a quantum well structure is formed on the GaN-based compound semiconductor layer, not only to reduce the dislocation density of the GaN-based compound semiconductor, but also to ensure the alleviation of its distortion. Because InGaN is not as hard as GaN, it can alleviate the distortion of the GaN-based compound semiconductor. Meanwhile, the superlattice layer having a quantum well structure can also alleviate the distortion of the GaN-based compound semiconductor because it has large distortion due to lattice mismatching.
Further, the GaN-based compound semiconductor device of the present invention comprises a buffer body which is formed discretely on a substrate or a buffer body having multiple pores which is formed on a substrate; a buffer layer formed on the buffer body; and a GaN-based compound semiconductor layer formed on the buffer layer.
In one embodiment of the present invention, this semiconductor device has a crystal nucleus generation-inhibiting layer between the substrate and the buffer body.
In another embodiment of the present invention, this semiconductor device has an InGaN layer or a superlattice layer having a quantum well structure on the GaN-based compound semiconductor layer.